From the Inform Designer's Manual:
Copyright on Inform, the program and its source code, its example games
and documentation (including this book) is retained by Graham Nelson,
who asserts the moral right to be identified as the author under the
Copyrights, Designs and Patents Act 1988. Having said this, I am happy
for it to be freely distributed to anybody who wants a copy, provided
that: (a) distributed copies are not substantially different from those
archived by the author, (b) this and other copyright messages are
always retained in full, and (c) no profit is involved. (Exceptions to
these rules must be negotiated directly with the author.) However, a
story file produced with the Inform compiler (and libraries) then
belongs to its author, and may be sold for profit if desired, provided
that its game banner contains the information that it was compiled by
Inform, and the Inform version number.